A variety of work machines such as, loaders, excavators, motor graders, and other types of construction, work and earth moving machinery use one or more hydraulically actuatable implements for accomplishing a task. These hydraulically actuatable implements may be operated by one or more hydraulic actuators such as a cylinder and a piston assembly that divides the cylinder into two chambers. The cylinder may be in fluid communication with a hydraulic pump for providing pressurized fluid to the chambers thereof, as well as in fluid communication with a fluid source or a tank for draining pressurized fluid therefrom. A valve arrangement may be connected between the pump and the cylinder and between the cylinder and the fluid source to control the flow rate and direction of the pressurized fluid to and from the chambers of the cylinder.
Each of the valve arrangements may include one or more electrically actuated compensated valves such as, independent metering valves (IMVs) that may be independently actuated to control the flow of pressurized fluid between the pump and the fluid source via the chambers of the cylinder. The amount of the pressurized fluid flowing to/from the cylinder may be controlled by changing the displacement of a valve spool in each valve. Each valve spool may typically include a series of metering slots that control the amount of fluid flowing through that valve. Changing the displacement of the valve spool may be accomplished by using an electrically controlled solenoid wound around an armature. When current is applied to the solenoid, the armature may be moved under electro-magnetic forces generated by the solenoid to cause the associated valve spool to displace a certain amount.
For precise flow control, the spool area needs to be controlled accurately. This is particularly true for compensated valves (such as IMVs) and electronically controlled pumps, because errors in spool area or pump flow may cause high pressure drops across a pressure compensator and affect the performance of the valve components such that each valve may perform differently from the others. Furthermore, hardware tolerances, such as actuator slope, springs, spool machining, and valve body machining may have a significant effect on the spool area. As a result, the valve components may not operate predictably and the performance of the cylinder may be degraded. Thus, in order to ensure a consistent performance of the cylinder and to accurately control the flow of pressurized fluid through the valves, a calibration technique is often utilized.
A prior art calibration technique is disclosed in the U.S. Pat. No. 7,997,117 assigned to the assignee of this Application. This prior art calibration technique utilizes a bypass valve to regulate pressure of pressurized fluid through the valves (IMVs) discussed above and adjusts the current being applied to the solenoids of those valves. It then uses a hardware characteristic map to determine a calibration for the valves. While effective, regulating pressure by utilizing the bypass valve is not as accurate as possible. Moreover, the calibration routine also does not have a method for determining the correct calibration current which is as robust as possible, leading to a calibration technique that is not entirely repeatable when tested on the work machine.
It would accordingly be beneficial if an improved technique of calibrating the hydraulic valves that overcomes at least some of the disadvantages of the prior art technique discussed above were developed.